FLOOD04661

• County: Boulder
• Community: Boulder
• Stream Name: South Boulder Creek
• Basin: South Platte
• Title: Floodplain Analyses of South Boulder Creek at Flatiron Property
• Date: 8/12/1996
• Prepared For: University of Colorado at Boulder
• Prepared By: Love & Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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HYDROLOGY

South Boulder Creek has a tributary drainage area of approximately 132 square miles at
its confluence with Boulder Creek, From its headwaters at the Continental Divide,
South Boulder Creek flows in an easterly direction through relatively steep, narrow
mountain canyons for nearly 22 miles whereupon it emerges onto the high plains at
Eldorado Springs, Colorado. From Eldorado Springs, the creek flows northeasterly
through a moderately wide valley over a distance of about 9.7 miles to its confluence
with Boulder Creek.

Gross Reservoir is a water storage facility located on South Boulder Creek approximately
3.5 miles upstream of Eldorado Springs. The reservoir was constructed in 1955 and has
a storage capacity of 43,000 acre-feet. The reservoir is operated by the Denver Water
Department and provides conservation storage for municipal use. It is not known
whether Gross Reservoir has any flood storage capacity available. A U.S. Geological
Survey stream gauging station is situated approximately 6.7 miles downstream of the
reservoir. The drainage area at the gauging station is approximately 109 square miles,

The impact of Gross Reservoir was evaluated by G&O by routing storm runoff through
the reservoir pool. The G&O study modeled South Boulder Creek using the EP A
Stormwater Management Model (SWMM). Flood peaks along South Boulder Creek
were defined in SWMM using a 6-hour duration storm event. The 100-year flood of
South Boulder Creek was routed in SWMM through Gross Reservoir, located upstream
of the study area, which significantly reduced downstream flood peaks (G&O, 1986).
The 100-year flood peak at U,S. Highway 36 is reported to be 6,160 cfs.

The peak flood of record at the gauging station downstream of Gross Reservoir occurred
in 1938 and was estimated to be 7,390 cubic feet per second (cfs) , However, it is also
important to note that the gauge was not operational in 1894 when major flooding
occurred in the Boulder Area and in the South Platte River basin.

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